1. Field of the Invention
The present invention relates to a light emitting diode with high luminance and method for making the same, and more particularly to a light emitting diode having a transparent window layer which is formed by a semiconductor film of nitrogen-containing compounds.
2. Description of the Related Art
Researches on nitrides of Group III or V attract people""s attention recently, particularly when the blue light emitting diode with high luminance was mass produced by a Japanese company in 1993, many companies become engaged in the development of blue and green light emitting diodes with high luminance and laser diodes.
In a semiconductor made of nitrogen-containing compounds of Group III or V, for an InGaAlP which energy gap less than that of nitride, when the ratio of aluminum to gallium in the active region of the light emitting element is changed, the light wavelength varies from 680 nm to 550 nm. Further, since the lattice constant of InGaAlP can match the GaAs substrate perfectly, InGaAlP is suitable for the light emitting element used for visible light region.
As shown in FIG. 1, the structure of a conventional InGaAlP light emitting diode is formed by growing a multi-layered epitaxial layer on a n-type GaAs light-absorbing substrate 10. First, a n-InGaAlP confining layer 11 is grown on the GaAs substrate 10, then a InGaAlP active layer 12 is grown on the n-InGaAlP confining layer 11 and then a p-InGaAlP confining layer 13 is grown on the active layer 12 such that a double heterostructure is formed. Finally, the light emitting surface of the diode is coated with a front surface metal electrode 14 and the surface of the GaAs substrate 10 on which the epitaxial layer is not formed is coated with a back surface metal electrode 15. The light wavelength of the light emitting diode is determined by the ratio of aluminum to gallium in the active layer 12. An increase in the amount of aluminum in the active layer may shorten the light wavelength of the light emitting diode. Meanwhile, the amount of aluminum in the confining layers 11, 13 outside the active layer 12 must be greater than that of aluminum in the active layer 12 such that the carriers may be not only effectively injected into the active layer 12 but also prevent the light emitted from the active layer 12 from being absorbed by the confining layers 11 and 13.
Generally speaking, with the exception of the combination efficiency of the electron and the electric hole, the major factor for determining the light emitting efficiency of a light emitting diode is whether the current on the front surface electrode 14 can be effectively distributed to the boundary of grains such that the light can be uniformly produced from the p-n junction. If the p-type confining layer 13 is too resistant to effectively distribute the current, the current will flow from the front surface electrode 14 to the back surface electrode 15, causing current jamming such that the current can not be spread effectively and that most of the generated light can not emit due to the blocking of the opaque front surface electrode 14 or the light emitting efficiency thereof is reduced because the generated light is reflected by the metal electrode 14 and absorbed by the substrate. The conventional InGaAlP light emitting diode has the above drawbacks. This is because the concentration of the p-type InGaAlP confining layer 13 is generally below 1xc3x971018 cmxe2x88x923 and the hole mobility thereof is merely 10xcx9c20 cm2xc2x7V/sec, which forms the resistivity 0.5 xcexa9-cm. The resistivity is so high that the lateral current can not be spread over the whole grain. To solve this problem, different structures, as shown in FIGS. 2 and 3, have been developed. The structure as illustrated in FIG. 2 is formed by growing a semiconductor window layer 16 differing from the InGaAlP layer on the p-type InGaAlP confining layer 13. The window layer is characterized by its low resistivity, perfect conductivity, and energy gap which is greater than that of the active layer 12. Thus, the semiconductor window layer 16 will not absorb the light emitted from the p-n junction. Generally speaking, the materials suitable for the semiconductor window layer 16 include GaAlAs, GaAsP, and GaP, etc. The optimum thickness for the window layer 16 ranges from five to tens of micrometers. However, the lattice constants of GaAsP and GaP do not match with those of the GaAs substrate and the InGaAlP layer 13, thus a large amount of dislocation on the growing interface is occurred and opto-electronic characteristics of the light emitting diode are adversely effected.
Another structure of the conventional technique is illustrated in FIG. 3 and is characterized in that a current blocking layer 17 and a Bragg reflector layer 18 are additionally formed and the current spreading layer or window layer 19 is made of GaAlAs. The current flowed from the front surface electrode 14 is thus influenced by the current blocking layer 17 so as to effectively distribute to the current spreading layer 19. In addition, the added Bragg reflector layer 18 between the GaAs substrate 10 and InGaAlP confining layer 11 can reduce the amount of light emitted from the InGaAlP active layer 12 to be absorbed by the GaAs substrate 10. Such a structure can double the light emitting efficiency of the light emitting diode. However, such a structure has drawbacks of complicated process and long processing time. The current blocking layer 17 is defined after two times of organic metal chemical vapor phase deposition and one time of masking and etching process. In addition, the aluminum ratio in the InGaAlP confining layer 13 is very high and thus oxidization is easily occurred and the growth is hard to control. Furthermore, the growth for the composition and the thickness of the Bragg reflector layer 18 should be precisely controlled and the thickness is about a few micrometers. Therefore, the manufacturing process will incur a much longer time.
As mentioned above, nitride has been applied to the manufacture of blue and green light emitting diodes. The emitting wavelength can be adjusted from a beam of violet light to a beam of green-blue light and even a beam of orange light by adjusting the metal components of Group III in the active layer. The conventional nitride researches are confined to the adjustment of the metal components of Group III and the manufacture of the blue and green light emitting diodes. Recently, nitride researches regarding the adjustment of the metal components of Group V, such as GaPN, GaAsN, GaPAsN, have been engaged.
Although the energy gaps of GaN and GaP are 3.4 eV and 2.3 eV respectively, the energy gap of is not increased with the increase amount of nitrogen but is increased with a bow-like curve. Therefore, when GaP1xe2x88x92xNx contains a few amount of nitrogen, it possesses an energy gap less than that of GaP. As far as GaP1xe2x88x92xxe2x88x92yAsxNy is concerned, the lattice constant of compound is reduced when the amount of nitrogen is increased and is increased when the amount of arsenic is increased. Such a characteristic can be used to adjust the lattice constant of GaP1xe2x88x92xxe2x88x92yAsxNy to be equal to, greater or smaller than that of GaP. Therefore, GaP1xe2x88x92xxe2x88x92yAsxNy can be used to replace the GaP window layer of a light emitting diode with high luminance so as to reduce dislocation and to increase light emitting efficiency.
To avoid the above-mentioned problems encountered in the prior art, the object of the present invention is to provide a novel high luminance InGaAlP light emitting diode having nitrogen-containing compounds and method for making the same. The present invention is mainly directed to growing a window layer of a light emitting diode with a nitrogen-containing compound on the double heterostructure of InGaAlP. Since the energy gap of the nitrogen-containing compound is greater than that of the light emitted from the active layer and is smaller than that of GaP, it is easily to be doped and to form metallic ohmic electrode. Therefore, it is suitable to form a window layer, thereby increasing the light emitting efficiency of a light emitting diode.